In the offshore oil and gas industry fluid communication between a subsea location and a surface vessel, such as a FPSO vessel, is achieved via conduit systems called risers. Many riser configurations exist, such as catenary risers, top tensioned risers and hybrid risers.
A hybrid riser typically consists of a near vertical bundle of metallic pipes composed of a plurality of outer conduits circumferentially arranged around a larger diameter central structural pipe that is typically evacuated and sealed. The outer pipes are often referred to as peripheral lines and are used for fluid communication along the length of the riser bundle. The lower end of the bundle is anchored, via the central structural pipe, at the seabed and terminated near the water surface by a large buoyant structure, such as an aircan which provides tension and thus supports the bundle of pipes in a manner to resist the applied environmental loads. At the top end of the riser bundle each peripheral line is connected to a surface vessel by a flexible pipe jumper configured in free hanging catenary geometry.
An advantage of the hybrid riser bundle over other riser configurations is that the bundle section is largely static and unaffected by wave and vessel motions due to the location of the top section below the wave zone and the use of flexible jumpers that largely isolate vessel motions from the bundle section.
A further benefit of the hybrid bundle is that it can be designed to be neutrally buoyant, beach fabricated and installed using a tow out and upending installation method. Neutral buoyancy is achieved through the application of syntactic foam modules along the entire riser length to support the steel peripheral lines. Such use of syntactic buoyancy modules is illustrated in FIG. 1 which is a lateral cross section of a portion of a known hybrid bundle 10. The bundle 10 includes a large diameter central structural pipe 12 which is surrounded by smaller diameter peripheral lines 14. Syntactic foam segments or modules 16 are provided to completely encase the pipe 12 and lines 14.
Following installation the riser can free stand by virtue of its syntactic buoyancy and supporting aircan following which the flexible jumpers can be installed and connected between the hybrid bundle and surface vessel.
As noted above, known hybrid riser systems incorporate syntactic buoyancy material, such as is illustrated in FIG. 1. In addition to providing buoyancy such material is also considered to insulate the peripheral lines. Although a degree of insulating may be achieved, this can be insufficient due to hot water convection losses in the water gap 18 (FIG. 1) between the lines 14 and the buoyancy material 16 and as such specialised insulation materials are often needed in addition to the syntactic buoyancy. Furthermore, the syntactic buoyancy material is very expensive with few international suppliers.
A further disadvantage is that the weight of the peripheral lines is such that the upper aircan becomes excessively large, particularly in deep water, such that it is difficult to fabricate, launch and handle during installation.
Another disadvantage of known hybrid riser systems is the problem associated with the structural interaction between the peripheral lines and the central structural member. Typically the peripheral lines are suspended from the top end of the bundle and allowed to hang down and slide axially with respect to the central structural pipe. As such the peripheral lines will normally have a maximum tension at their top end due to self weight and zero tension at their lower end. At the top end of each peripheral line their weight is reacted into the central pipe and whilst the effective tension across the section may be modest the true wall tension in the wall of the central structural pipe may be prohibitive such that this may limit the weight and number of peripheral lines which are feasible. This has a serious impact on the applicable water depth.
The problem of peripheral line axial movement with respect to the central pipe is further compounded when the bundle section is flexed and the resultant changes in curvature cause further relative axial motion between the central pipe and peripheral lines. Due to the high axial stiffness of the peripheral lines it is not practical to constrain this axial movement due to the resulting high tension and compression generated that can damage buoyancy modules and other fittings.
A further problem resulting from this known method of supporting the peripheral lines is that of relative expansion with respect to the central pipe. The peripheral lines expand and contract due to pressure and temperature effects such that the bottom end of the peripheral lines moves axially with respect to the central pipe. In deep water this may be a significant movement in the order of 1-5 m and it is recognised by those skilled in the art that this is a difficult problem to resolve requiring large and complex jumper pipes with sufficient flexibility to accommodate the movement.
These and other design issues increase in complexity with depth and the number of peripheral lines such that it is difficult to economically use this technology beyond certain water depths.
U.S. Pat. No. 6,082,391 describes an example of a known hybrid riser system.